1. Field of the Invention
The present invention relates to a display device using a light emitting element in a pixel portion. More particularly, the invention relates to a display device using a light emitting element typified by an organic electroluminescence (EL) element in a pixel portion and having a video data correction circuit for correcting video data in accordance with the degradation of the light emitting element. Further, the invention relates to a display device that has a display panel where a light emitting element such as an EL element is provided in each pixel, a control circuit having a storing means for storing video data, and a video data correction circuit for correcting the degradation of the light emitting element.
2. Description of the Related Art
As a substitute for an LCD (Liquid Crystal Display), there is a display device that is constituted by a display panel having a light emitting element in each pixel and a peripheral circuit for inputting a signal to the panel, and that displays images by controlling light emission of the light emitting element.
Such a display device includes a control circuit for outputting to a display panel a panel control signal as well as video data obtained by converting a received video signal so as to achieve gray scale display in a pixel of the panel. In the display panel of the display device, typically two or three TFTs (Thin Film Transistors) are provided in each pixel, and a current supplied to a light emitting element in each pixel, that is, the luminance and light emission/non-light emission of the light emitting element in each pixel are controlled by controlling on/off of these TFTs. In addition, a driver circuit for controlling on/off of the TFTs in each pixel is provided at the periphery of the pixel portion of the panel. This driver circuit may be constituted by TFTs that are formed at the same time as the TFTs in the pixel portion. These TFTs may be either N-channel TFTs or P-channel TFTs.
Gray scale display in the pixel having the aforementioned configuration is performed typically by an analog method or a digital method. The digital method is advantageous in that it is not influenced by variations in characteristics of TFTs. Known as a digital gray scale display method are a time gray scale method and an area gray scale method.
According to the time gray scale method, gray scale display is performed by controlling a period during which each pixel of a display device emits light. When it is assumed that one image is displayed during one frame period, one frame period is divided into a plurality of subframe periods. Light emission or non-light emission of each pixel is selected for each subframe period (that is, a light emitting element in each pixel emits light or no light), and each subframe period is weighted (that is, each subframe period has a different display period). The accumulated light emitting periods are controlled by selecting the subframe periods (that is, by selecting a combination of subframe periods during which a pixel emits light), thereby gray scale display in each pixel can be performed.
According to the area gray scale method, gray scale display is performed by controlling an area that emits light in each pixel of a display device. Specifically, each pixel is divided into subpixels and the number of subpixels that emit light is changed, thereby gray scale display in each pixel can be performed.
When a light emitting element such as an EL element is used, a current is always supplied to the EL element to flow therethrough during a period when the EL element emits light. Accordingly, the EL element itself degrades when it emits light for a long period, which causes variations in luminance characteristics. That is to say, an EL element that has degraded and an EL element that has not degraded have different luminance even when a current is supplied from the same current source at the same voltage.
Therefore, some display devices using a light emitting element such as an EL element include a video data correction circuit in order to maintain the uniformity of a screen while preventing luminance variations even when an EL element in a certain pixel degrades. The video data correction circuit detects the lighting time or the lighting time and lighting intensity of each pixel by periodically sampling a video data signal, and compares the detected accumulation value to previously stored data on changes with time of luminance characteristics of the EL element. As a result, it is possible to correct the video data signal for driving a pixel including an EL element that has degraded.
As such a display device, there is, for example, a self-light emitting display device having a degradation correction function, which is disclosed in Patent Document 1. FIG. 10 is a block diagram of a degradation correction device. The degradation correction device shown in FIG. 10 is constituted by a counter portion I, a storage circuit portion II, and a signal correction portion III. The counter portion I includes a counter 1002, the storage circuit portion II includes a volatile memory 1003 and a nonvolatile memory 1004, and the signal correction portion III includes a correction circuit 1005 and a correction data storage portion 1006. In this degradation correction device, video data for driving a pixel including an EL element that has degraded, which is included in a first video signal 1001A that is a video data signal before being corrected, is corrected by the signal correction portion III and supplied to a display device 1007 as a second video signal 1001B that is a video data signal after being corrected.
In this degradation correction device, the first video signal 1001A is sampled periodically (e.g., every second), and light emission or non-light emission of each pixel is counted by a counter 1002 depending on the sampled signal. The counted number of lighting times in each pixel, namely an accumulated lighting time (hereinafter referred to as accumulated time data) is sequentially stored in the storage circuit portion II. The storage circuit is desirably configured by using a nonvolatile memory since the number of lighting times is accumulated. However, the number of writing times to a nonvolatile memory is generally limited; therefore, in the device shown in FIG. 10, the data is stored in the volatile memory 1003 during operation of the self-light emitting device and written to the nonvolatile memory 1004 periodically (e.g., every hour, or when the power is turned off). That is to say, the lighting time or the lighting time and lighting intensity of the EL element are counted continuously the next time the power is turned on.    [Patent Document 1] Japanese Patent Laid-Open No. 2002-175041